Observed changes of temperature extremes during 1960�C2005 in China: natural or human-induced variations?

The purpose of this study was to statistically examine changes of surface air temperature in time and space and to analyze two factors potentially influencing air temperature changes in China, i.e., urbanization and net solar radiation. Trends within the temperature series were detected by using Mann-Kendall trend test technique. The scientific problem this study expected to address was that what could be the role of human activities in the changes of temperature extremes. Other influencing factors such as net solar radiation were also discussed. The results of this study indicated that: (1) increasing temperature was observed mainly in the northeast and northwest China; (2) different behaviors were identified in the changes of maximum and minimum temperature respectively. Maximum temperature seemed to be more influenced by urbanization, which could be due to increasing urban albedo, aerosol, and air pollutions in the urbanized areas. Minimum temperature was subject to influences of variations of net solar radiation; (3) not significant increasing and even decreasing temperature extremes in the Yangtze River basin and the regions south to the Yangtze River basin could be the consequences of higher relative humidity as a result of increasing precipitation; (4) the entire China was dominated by increasing minimum temperature. Thus, we can say that the warming process of China was reflected mainly by increasing minimum temperature. In addition, consistently increasing temperature was found in the upper reaches of the Yellow River basin, the Yangtze River basin, which have the potential to enhance the melting of permafrost in these areas. This may trigger new ecological problems and raise new challenges for the river basin scale water resource management.     

Thresholds of hydrologic flow regime of a river and investigation of climate change impact—the case of the Lower Brahmaputra river Basin

The sustainability of social-ecological systems depends on river flows being maintained within a range to which those systems are adapted. In order to determine the extent of this natural range of variation, we assess ecological flow thresholds and the occurrence of potentially damaging flood events to society in the context of the Lower Brahmaputra river basin. The ecological flow threshold was calculated using twenty-two ‘Range of Variability (RVA)’ parameters, considering the range between?±?1 standard deviation from the mean of the natural flow. Damaging flood events were calculated using flood frequency analysis of Annual Maxima series and using the flood classification of Bangladesh. The climate change impacts on future river flow were calculated by using a weighted ensemble analysis of twelve global circulation models (GCMs) outputs driving a large-scale hydrologic model. The simulated climate change induced altered flow regime of the Lower Brahmaputra River Basin was then investigated and compared with the calculated threshold flows. The results demonstrate that various parameters including the monthly mean of low flow (January, February and March) and high flow (June, July and August) periods, the 7-day average minimum flow, and the yearly maximum flow will exceed the threshold conditions by 1956–1995 under the business-as-usual A1B and A2 future scenarios. The results have a number of policy level implications for government agencies of the Lower Brahmaputra River Basin, specifically for Bangladesh. The calculated thresholds may be used as a good basis for negotiations with other riparian countries of the basin. The methodological approach presented in this study can be applied to other river basins and provide a useful basis for transboundary water resources management.     

Effect of Counter-Gradient in the Computation of Turbulent Fluxes of Heat and Moisture in a Regional Model

The counter-gradient terms in the computations of turbulent fluxes of heat and moisture have been included in the PBL parameterization of a regional model for monsoon prediction. Results show that inclusion of counter-gradient terms has a marginal impact in the prediction of large scale monsoon circulation and rainfall rates.     

The inter-annual variations and the long-term trends of monthly air temperatures in Iraq over the period 1941–2013

Theoretical and Applied Climatology - Mean annual and monthly averages of mean (Tmean), maximum (Tmax) and minimum (Tmin) air temperature from seven stations in Iraq were analysed to detect the...     

Radiation balance at the surface in the city of S?o Paulo, Brazil: diurnal and seasonal variations

The main goal of this work is to describe the diurnal and seasonal variations of the radiation balance components at the surface in the city of S?o Paulo based on observations carried out during 2004. Monthly average hourly values indicate that the amplitudes of the diurnal cycles of net radiation (Q*), downwelling and upwelling shortwave radiation (SW_DW, SW_UP), and longwave radiations (LW_DW, LW_UP) in February were, respectively, 37%, 14%, 19%, 11%, and 5% larger than they were in August. The monthly average daily values indicate a variation of 60% for Q*, with a minimum in June and a maximum in December; 45% for SW_DW, with a minimum in May and a maximum in September; 50% for SW_UP, with a minimum in June and a maximum in September; 13% for LW_DW, with a minimum in July and a maximum in January; and 9% for LW_UP, with a minimum in July and a maximum in February. It was verified that the atmospheric broadband transmissivity varied from 0.36 to 0.57; the effective albedo of the surface varied from 0.08 to 0.10; and the atmospheric effective emissivity varied from 0.79 to 0.92. The surface effective emissivity remained approximately constant and equal to 0.96. The albedo and surface effective emissivity for S?o Paulo agreed with those reported for urban areas in Europe and North America cities. This indicates that material and geometric effects on albedo and surface emissivity in S?o Paulo are similar to ones observed in typical middle latitudes cities. On the other hand, it was found that S?o Paulo city induces an urban heat island with daytime maximum intensity varying from 2.6°C in July (16:00 LT) to 5.5°C in September (15:00 LT). The analysis of the radiometric properties carried out here indicate that this daytime maximum is a primary response to the seasonal variation of daily values of net solar radiation at the surface.     

Constructions and Applied Examinations of a Kind of Square-Conservative Schemes in High Precision in the Time Direction

In order to meet the needs of work in numerical weather forecast and in numerical simulations for climate change and ocean current, a kind of difference scheme in high precision in the time direction developed from the completely square-conservative difference scheme in explicit way is built by means of the Taylor expansion. A numerical test with 4-wave Rossby-Haurwitz waves on them and an application of them on the monthly mean current the of South China Sea are carried out, from which, it is found that not only do the new schemes have high harmony and approximate precision but also can the time step of the schemes be lengthened and can much computational time be saved. Therefore, they are worth generalizing and applying.     

Observed variability and trends in extreme temperature indices and rice–wheat productivity over two districts of Bihar, India—a case study

The purpose of this paper is to analyze the trends and variability in extreme temperature indices and its impact on rice–wheat productivity over two districts of Bihar, India, which is part of the middle Indo-Gangetic Basin. Mann–Kendall non-parametric test was employed for detection of trend and Sen slope was determined to quantify the magnitude of such trends. We have analyzed 10 extreme temperature indices for monthly and seasonally. The influence of extreme temperature indices on rice–wheat productivity was determined using correlation analysis. As far as Patna is concerned, if the number of cool days during September ≥10, the rice productivity will increase due to the availability of sufficient duration to fill up the grain. However, higher warm days during all the months except June will affect the productivity. A significant negative correlation was noticed between maximum value of minimum temperature during September and rice productivity. Highly significant positive correlation was noticed between number of cool days during September with rice productivity while it was highly significant negative correlation in the case of number of warm days during the same month. As far as Samastipur is concerned, a negative correlation was noticed between wheat productivity and maximum value of maximum temperature (TXx) during February, but not statistically significant. The higher temperature may affect the kernel weight and thereby yield. It is seen that a critical value of TXx ≥29.2 °C will be harmful to wheat crop during February. A significant positive correlation of number of cool nights with wheat productivity also supports the above relationship. The critical values of extreme temperature indices during rice and wheat growing months provide an indicator to assess the vulnerability of rice–wheat productivity to temperature for Patna and Samastipur districts and there is a need to prepare an adaptive strategy and also develop thermo-insensitive rice–wheat high yielding varieties suitable for this region to sustain rice–wheat productivity under projected climate change situation.     

Low-frequency oscillations of the East Asia–Pacific teleconnection pattern and their impacts on persistent heavy precipitation in the Yangtze–Huai River valley

Based on the daily reanalysis data from NCEP NCAR and daily precipitation data from the China National Meteorological Information Center,an ensemble empirical mode decomposition method is employed to extract the predominant oscillation modes of the East Asia Pacific(EAP) teleconnection pattern.The influences of these low-frequency modes on persistent heavy precipitation in the Yangtze Huai River(YHR)valley are investigated.The results indicate that the EAP pattern and rainfall in YHR valley both exhibit remarkable 10 30- and 30 60-day oscillations.The impacts of the EAP pattern on the YHR persistent heavy precipitation can be found on both the 10 30- and 30 60-day timescales the 10 30-day scale for most cases.Composite analysis indicates that,on the 10 30-day timescale,formation of the EAP pattern in the lower and middle troposphere is determined by convective systems near the tropical western Pacific;whereas in the middle troposphere,the phase transition is jointly contributed by both the dispersion of zonal wave energies at higher latitudes and convective systems over the South China Sea.In the context of the10 30-day EAP pattern,the anomalously abundant moisture is transported by an anomalous subtropical anticyclone system,and strong moisture convergence results from that anomalous anticyclone system and a cyclonic system in the midlatitude East Asia.Such a combination of systems persists for at least three days,contributing to the formation of persistent heavy precipitation in the YHR valley.     

Initialisation and predictability of the AMOC over the last 50 years in a climate model

The mechanisms involved in Atlantic meridional overturning circulation (AMOC) decadal variability and predictability over the last 50 years are analysed in the IPSL–CM5A–LR model using historical and initialised simulations. The initialisation procedure only uses nudging towards sea surface temperature anomalies with a physically based restoring coefficient. When compared to two independent AMOC reconstructions, both the historical and nudged ensemble simulations exhibit skill at reproducing AMOC variations from 1977 onwards, and in particular two maxima occurring respectively around 1978 and 1997. We argue that one source of skill is related to the large Mount Agung volcanic eruption starting in 1963, which reset an internal 20-year variability cycle in the North Atlantic in the model. This cycle involves the East Greenland Current intensity, and advection of active tracers along the subpolar gyre, which leads to an AMOC maximum around 15 years after the Mount Agung eruption. The 1997 maximum occurs approximately 20 years after the former one. The nudged simulations better reproduce this second maximum than the historical simulations. This is due to the initialisation of a cooling of the convection sites in the 1980s under the effect of a persistent North Atlantic oscillation (NAO) positive phase, a feature not captured in the historical simulations. Hence we argue that the 20-year cycle excited by the 1963 Mount Agung eruption together with the NAO forcing both contributed to the 1990s AMOC maximum. These results support the existence of a 20-year cycle in the North Atlantic in the observations. Hindcasts following the CMIP5 protocol are launched from a nudged simulation every 5 years for the 1960–2005 period. They exhibit significant correlation skill score as compared to an independent reconstruction of the AMOC from 4-year lead-time average. This encouraging result is accompanied by increased correlation skills in reproducing the observed 2-m air temperature in the bordering regions of the North Atlantic as compared to non-initialized simulations. To a lesser extent, predicted precipitation tends to correlate with the nudged simulation in the tropical Atlantic. We argue that this skill is due to the initialisation and predictability of the AMOC in the present prediction system. The mechanisms evidenced here support the idea of volcanic eruptions as a pacemaker for internal variability of the AMOC. Together with the existence of a 20-year cycle in the North Atlantic they propose a novel and complementary explanation for the AMOC variations over the last 50 years.     

Interannual variations of the winter‐time outcrop area of subtropical mode water in the western north pacific ocean

Abstract

Seasonal and interannual variations of the SST 16–19°C zone in the western North Pacific are described. Temperatures ranging from 16 to 19°C correspond with those of the Subtropical Mode Water (SMW) first reported and named by Masuzawa (1969). In the cooling season, this zone gradually moves southward and about December crosses latitudes 35–37°N where the Kuroshio axis lies. From January to April, the zone stagnates and spreads from the Kuroshio axis to about 28°N, i.e. to a width of about 700 km at 145°E in midwinter. This stagnation and widening are a manifestation of the existence of a thick mixed layer of SMW, i.e. the formation of a large amount of SMW, which is confirmed by several examples of the subsurface temperature distribution. In the heating season, the zone migrates northward with a narrow width as a result of the warming of the surface layer through the air‐sea interface. SST maps in March, and other related data, show the large interannual variations of the zone, especially in the sea west of the Izu Ridge.     

Analysis of deep‐drogued satellite‐tracked drifter measurements in the northeast pacific

Abstract

We analyse the trajectories of 24 deep‐drogued, satellite‐tracked drifters launched between 50 and 52°N in the northeast Pacific during June and October 1987. Three aspects of the observed motions at the drogue depths of 100 to 120 m are studied: (i) the spatial structure of the mean and variance velocity fields; (ii) the dispersion and eddy diffusion characteristics of the fluctuating motions; and (iii) the properties of selected mesoscale eddies.

The mean Lagrangian velocity field is consistent with the mean flow pattern derived from the historical dynamic height topography. Fluctuating motions within the region are dominated by mesoscale eddies and meanders. Several instances of persistent O(100 days) westward flowing countercurrents were also observed. Based on the Lagrangian integral time‐ and length scales, drifter motions become decorrelated within a period of 10 days and a separation of 100 km. The mean zonal and meridional integral time‐scales of 4.5 and 3.6 days, respectively, are nearly identical with those obtained by Krauss and Böning (1987) from deep‐drogued drifter tracks in the North Atlantic. Because of the relatively small (<100 cm² s^?2) kinetic energy values in the northeast Pacific, the corresponding mean Lagrangian length scales of 29.4 and 29.9 km are roughly half those for the Atlantic.

The observed drifter dispersion is generally consistent with Taylor's (1921) theory for single‐particle dispersion in homogeneous isotropic turbulence. Estimates obtained using 476 pseudo‐drifter tracks generated from the original records indicate that the dispersion increases linearly with time, t, within the first 3 to 5 days of launch and subsequently increases as t^1/2 (the random‐walk regime) within 10 days of launch. The respective peak zonal and meridional eddy diffusion coefficients of 4.1 × and 3.8 × 10⁷ cm² s^?1 are reached within 30 days of deployment. Similar estimates for the peak eddy diffusivities are obtained using dispersion curves for sets of 4 drifters launched at the same location during the June and October deployments. The dispersion of these clusters followed an exponential rather than a t^1/2 dependence over the first 70 days after release.

Eddies are predominantly clockwise rotary and are characterized by radii of 26 ± 16 km, periods of rotation of 16.0 ± 5.2 days, and azimuthal current speeds of 12.7 ± 8.6 cm s^?1. One eddy was tracked for over 10 months. Oceanographic data collected during the October deployment period showed the eddies have vertical extents of 500 to 700 m and are linked to isotherm depressions of over 100 m in the main pycnocline. All eddies in the bifurcation zone propagate to the west at roughly 1.5 ± 0.4 cm s^?1 counter to the prevailing mean flow and winds. These speeds are consistent with the westward phase speeds of first mode baroclinic planetary (Rossby) waves.     

Spatial and temporal variations of global frictional torque during the period 1948–2011

Frictional torque is an important mean for momentum exchange between the atmosphere and earth, and significantly influences the variation in atmospheric angular momentum. Using NCEP-NCAR reanalysis data for the period 1948–2011, we examined the spatial and temporal patterns of frictional torque. It was found that the globally integrated frictional torque turned from positive to negative in 1972, suggesting that angular momentum was transferred from the earth to the atmosphere before 1972, but from the atmosphere to the earth thereafter. The global frictional torque steadily declined from 1948 to 1994, but has been increasing since 1995. It was also found that the global frictional torque is mainly determined by the wind systems in the mid and low latitudes of the Southern Hemisphere (SH), where large changes in frictional torque occurred during the study period. Westerly wind increased continuously in the midlatitudes after 1948, while easterly wind decreased in the tropics of the SH after the 1980s.     

Climatological features of glacier and valley winds at the Hintereisferner (Ötztal Alps,Austria)

Summary Homogeneous wind measurements during summer 1971 and the 2 years 1977/78 were analysed at 3 sites of Hintereisferner (HEF) which is a valley-type glacier of 9 km length and northeasterly exposition in the Austrian Ötztal Alps. Some manifestations of glacier winds were found to verify a mesoscale circulation driven by gravity and differential heating of the air above ice surfaces and their ice free moraine surroundings. Modifications are mainly due to local topography and gradient winds.Throughout the year the wind regime at the glacier, esp. at the tongue, is clearly dominated by downsloping winds, reflecting the great potential of snow and ice areas in generating cold air downflow. Undisturbed glacier winds were found to occur most likely on sunny days with weak upper air winds. An influence of katabatic winds down from surrounding moraine slopes is indicated during night time hours. During sunlit hours the occurrence and strength of glacier winds is clearly correlated to the seasonal and daily solar cycle. The development of a regular diurnal variation of wind speeds with a single maximum about 5 m/s during afternoon hours is typical for the glacial wind regime and is most pronounced during the melting season. The observed wind speeds correspond with the diurnal development of vertical and horizontal temperature gradients of the air above the glacier.Clear day northerly winds penetrate most frequently in spring and autumn as far as to the tongue of HEF and are likely to represent thermally driven upvalley winds. They characterize fine weather in alpine valleys, when even signs of a local slope circulation above excessively heated moraine surfaces are indicated too.With 14 Figures     

Analysis of a regional change in the sign of the SAM�Ctemperature relationship in Antarctica

This study examines regional atmospheric circulation changes associated with a reversal in the sign of the relationship between the Southern Annular Mode (SAM) and near-surface temperatures at Halley station, East Antarctica, during the 1980s. We show that the key factor affecting the regional SAM?Ctemperature relationship (STR) is the relative magnitude of two climatological low pressure centres to the west and east of the area, which determines the source region of air masses advected into the locality. The principal difference affecting the STR is shown to be a trend towards a significantly weaker climatological low (higher pressure) at ~20°E during a positive phase of the SAM. Specifically, it is variations in the phase and magnitude of the wave number three patterns of atmospheric circulation, the non-annular component of the SAM, which are the principal factors governing the regional STR. A similar reversal is observed in the sign of the correlation between the SAM and oxygen-isotope values from an ice core located some 1,200?km east of Halley. This relationship is examined throughout the 20th Century, by comparing the isotope data to SAM reconstructions, and demonstrates marked decadal variability. Thus, these data suggest that switches in the STR are more likely to reflect natural variability in the long-wave patterns over the Southern Ocean rather than the influence of an anthropogenic forcing. This finding is important when considering the potential utility of Antarctic isotope data as a proxy for the SAM.     

Classification and Diurnal Variations of Precipitation Echoes Observed by a C-band Vertically-Pointing Radar in Central Tibetan Plateau during TIPEX-Ⅲ 2014-IOP

This study investigates classification and diurnal variations of the precipitation echoes over the central Tibetan Plateau based on the observations collected from a C-band vertically-pointing frequency-modulated continuous-wave(C-FMCW)radar during the Third Tibetan Plateau Atmospheric Scientific Experiment(TIPEX-Ⅲ)2014-Intensive Observation Period(2014-IOP).The results show that 51.32%of the vertical profiles have valid echoes with reflectivity–10 dBZ,and 35.06% of the valid echo profiles produce precipitation at the ground(precipitation profiles);stratiform precipitation with an evident bright-band signature,weak convective precipitation,and strong convective precipitation account for 52.03%,42.98%,and 4.99% of the precipitation profiles,respectively.About 59.84% of the precipitation occurs in the afternoon to midnight,while 40.16% of the precipitation with weaker intensity is observed in the nocturnal hours and in the morning.Diurnal variation of occurrence frequency of precipitation shows a major peak during 2100–2200 LST(local solar time)with 59.02%being the stratiform precipitation;the secondary peak appears during 1300–1400 LST with 59.71% being the weak convective precipitation;the strong convective precipitation occurs mostly(81.83%)in the afternoon and evening with two peaks over 1200–1300 and 1700–1800 LST,respectively.Starting from approximately 1100 LST,precipitation echoes develop with enhanced vertical air motion,elevated echo top,and increasing radar reflectivity.Intense upward air motion occurs most frequently in 1700–1800 LST with a secondary peak in 1100–1400 LST,while the tops of precipitation echoes and intense upward air motion reach their highest levels during 1600–1800 LST.The atmospheric conditions in the early morning are disadvantageous for convective initiation and development.Around noon,the convective available potential energy(CAPE)increases markedly,convective inhibition(CIN)is generally small,and a super-dry-adiabatic layer is present near the surface(0–400 m).In the early evening,some larger values of CAPE,level of neutral buoyancy,and total precipitable water are present,suggesting more favorable thermodynamic and water vapor conditions.     

Observed temperature evolution in the City of Sfax (Middle Eastern Tunisia) for the period 1950–2007

This paper studies temperature evolution in the city of Sfax (Middle Eastern Tunisia, with more than 600 000 people) from 1950 to 2007. Daily maximum and minimum temperatures recorded at Sfax observatory from 1950 to 2007 are analysed by studying their homogeneity, possible trends and their statistical significance. Linear regression, Student and Mann–Kendall trend test were applied to annual mean minimum and maximum temperature data to determine the existence and significance of trends. Using a number of statistical tests, it is found that the data measured at the surface station represent a non homogenous time-series. Furthermore, mean annual and monthly temperatures are evaluated and a statistically significant trend starting from year 1950 was found. Important increase of the surface temperature in the City of Sfax was found after 1984. The increase in the surface temperature in the city of Sfax is further associated with global, regional (e.g. Mediterranean area) and meso-scale temperature increase. In addition, the spatial pattern of surface temperature in the city of Sfax from 1982 to 2007 shows that the overall land surface temperature increased with the expansion of Urban Heat Island (UHI) from urban areas to suburban districts.     

Temporal and spatial variations of high-impact weather events in China during 1959–2014

Theoretical and Applied Climatology - The variation and trend in the frequency and duration of four types of high-impact weather (HIW) events were examined using daily surface climate data and...     

Analysis of the interannual variations and influencing factors of wind speed anomalies over the Beijing–Tianjin–Hebei region

风场对京津冀地区雾霾的产生和消散起着决定性作用。本文利用站点观测数据,研究了京津冀地区冬季风场的年际变化及其影响因素。研究表明,京津冀地区的冬季平均风速为2.0 m s~(-1),每年降幅为0.01 m s~(-1)。大多数情况下,强风年对应热带太平洋东部的负海温异常,而弱风年份相反。此外,京津冀地区冬季风场的年际变化还受到包括北半球中高纬度气压梯度、欧亚大陆地表温度、菲律宾东部热带太平洋海面温度等多重因素的影响。